Athermal laser glass composition

ABSTRACT

A phosphate laser glass in which the optical path length is substantially independent of temperature, or &#34;athermal&#34;, particularly under rapid pulsing, and chemically stable, while providing high gain. The base glass composition comprises, in mole percent, 55 to 70% P 2  O 5 , 3 to 15% of an alkali metal oxide, preferably Li 2  O plus K 2  O, 20 to 30% BaO and 0.5 to 5% Al 2  O 3 . Up to 15% of the BaO may be replaced by CaO, SrO or MgO and part of the Al 2  O 3  may be replaced by Nd 2  O 3 . The total Al 2  O 3  plus Nd 2  O 3  is 4 mole percent ±1.5%. The laser glass composition also includes a laseable component, preferably Nd 2  O 3  and preferably includes a solarization inhibitor.

BACKGROUND OF THE INVENTION

The present invention relates to inorganic laser glass compositions,particularly phosphate laser glass compositions, which exhibit athermalbehavior under high thermal loading, good water and chemical durabilityand high gain.

Silicate laser glass compositions were developed to replace natural orsynthetic laser crystals for commercial and military applications. Morerecently, phosphate laser glass has generally replaced silicate laserglass in many applications because of the high gain potential ofphosphate systems and the inherent disadvantages of silicate systems, asdisclosed in U.S. Pat. No. 4,075,120 and the copending application ofthe assignee, Ser. No. 877,606, now U.S. Pat. No. 4,248,732.

Laser rods have been limited in their ability to generate high averagebrightness by thermally induced optical distortions. These distortionsappear as an increase in beam divergence, accompanied by depolarizationof the beam. The distortion of the beam appears to vary directly withthe input power to the laser and results in both a change in the cavity"Q" and a degradation and eventual failure of the laser to produce asingle pulse, where the laser includes a polarization-sensitive"Q"-switch. Temperature variations are necessarily created in a laserrod or the like during lasing; these temperature variations inducedistortions of the optical paths within the laser rod, especially whenaccentuated by periodic pulsing of the laser.

The prior art has attempted to develop "athermal" laser glasscompositions by balancing the constituents of the glass to obtain anegative temperature coefficient of refractive index, dn/dt, based uponthe following equation:

    W=dn/dt+α(n-1)

wherein, W is the thermo-optic constant, α is the coefficient of thermalexpansion and n is the index of refraction. The change in thethermo-optic constant in a laser rod based upon changes in temperature,is then, as follows:

    ΔW=Δt[dn/dt+α(n-1)]×L

wherein, Δt is the change in temperature and L is the length of thelaser rod or lasing body. Unfortunately, several other factors affectthe lasing properties of a laser rod or the like, including Young'smodulus, etc.

For example, the optical distortion of a plane wave P, averaged for twopolarizations, is given by the following formula: ##EQU1## wherein, E isYoung's modulus, μ is Poison's ratio, C₁ is the photoelastic constant ofthe glass, perpendicular to stress and C₂ is the photoelastic constantparallel to the stress.

The birefringence Q may be determined from the following equation:##EQU2## wherein, C₁ -C₂ is generally accepted as the stress-opticcoefficient.

Both experimental and theoretical studies have been performed relatingthese thermo-optic constants with lasing characteristics. For example,certain tests have established that the distortions of the optical pathsinside a lasing element is due to the inhomogeneous temperature field,which is strongly dependent on the average temperature in which theactive element is operated. For each type of glass, there is atemperature at which the values of the thermo-optic distortions in onepolarization are minimal and are governed only by the temperature dropinside the active elements and the temperature coefficient of thethermo-active constants, see Solid-State Laser Engineering, Koechner(1976) Springer-Verlag. The relation of the thermo-optic constant Qabove was reported in Sov. J. Opt. Technol. Mak, et al., Vol. 38, 553(1971) which concluded that the output energy per pulse decreases whenthe average pump power is increased and the rate of this decrease isdependent upon the thermo-optic constant Q, which represents thebirefringence of the glass.

As described above, each of the thermo-optic constants W, P and Q arecomposition and temperature dependent and each may be made individuallypositive, negative or theoretically zero. However, the combination of W,P and Q, acting together, determine the optical distortion of the laserglass at each temperature profile, within the laser rod. There existsfor each combination of the thermo-optic constants, a given temperatureprofile for which the optical distortion is a minimum.

Thus, contrary to the teaching of the prior art, it is not possible toachieve an athermal laser glass merely by adjusting one or moreconstituents to achieve a negative temperature coefficient of refractiveindex. The factors are too complex, requiring an experimental approachbased upon all of the known thermo-optic constants.

Another problem with many laser glass compositions suggested by theprior art has been poor water durability. In commercial applications,the temperature of the laser rod is reduced or maintained by a coolingfluid, generally water. The laser rod disclosed herein is water cooled.A laser rod must therefore be stable in the cooling medium.

SUMMARY OF THE INVENTION

As stated above, phosphate glass compositions have replaced silicatesfor laser rods in many applications, based upon the advantages ofphosphate glass laser rods, including a low laser threshold and chemicaldurability. Phosphate glass laser rods have not however achieved theiranticipated potential because of thermally induced optical distortionswhich have limited the ability of the rod to generate high averagebrightness. The prior art has attempted to balance the properties of theglass to achieve athermal behavior, however this effort has been onlypartially successful and at the expense of other equally importantproperties, particularly gain.

It has now been discovered that it is possible to develop a phosphatelaser glass which has high gain, athermal behavior or constant beamdivergence, good water or chemical durability and which is capable ofhandling high thermal loading. This has been accomplished by taking allof the thermo-optic constants into account and experimentallydetermining the relationship between the constituents which affect theseconstants.

In the broadest terms, the base phosphate laser glass of this inventioncomprises, in mole percent, 55 to 70% P₂ O₅, 3 to 15% of an alkali metaloxide, preferably Li₂ O plus K₂ O, 10 to 30% BaO, 0 to 15% CaO and 0 to15% SrO, wherein the total RO concentration is 20 to 28% and 0.5 to 5%Al₂ O₃. The concentrations of the constituents of the base glasscomposition are given in mole percent to permit one to one substitutionof equivalent constituents. In addition, the laser glass of thisinvention includes a laseable component, preferably Nd₂ O₃ and asolarization inhibitor. The laser glass composition of this inventionincludes 0.5 to 11 percent, by weight, Nd₂ O₃ and 1 to 5 percent, byweight, of a solarization inhibitor selected from the group consistingof Sb₂ O₃, Nb₂ O₅ and SiO₂. At least part of the Nd₂ O₃ may besubstituted for Al₂ O₃, such that the total Al₂ O₃ plus Nd₂ O₃ isbetween 2.5 and 5.5 mole percent.

The additions to the base glass composition, including the laseableconstituent and the solarization inhibitor, are given in weight percentof the total glass composition because such consituents have been addedto the base glass. This method of calculation has greatly facilitatedtesting of various base glass compositions to determine the affect ofvariations upon the thermo-optic constants. The preferred embodiment ofthe base phosphate laser glass of this invention include 6 to 12 molepercent Li₂ O₃. The most preferred embodiment includes 1 to 3 percent K₂O, 0.5 to 4 percent Al₂ O₃ and the total Al₂ O₃ plus Nd₂ O₃ is 4 molepercent ±1%.

The preferred laser glass compositions of this invention have an outputof 800 millijoules or greater at 40.5 joules input, when pulsed at onepulse per second. The most preferred embodiments have an output of over1000 millijoules with a threshold of less than 8 joules, when pulsed atone pulse per second. Chemical or water durability in the preferredcompositions is less than 6×10⁻⁶ grams/minute/cm² at 100° C. This is ameasurement of the loss in weight of a premeasured sample in boilingdistilled water after 60 minutes. Beam divergence was measured by firingthe laser through a lens against exposed film located at the focallength of the lens. Little or no beam divergence was measured at one,five and ten pulses per second in the preferred embodiments. That is,the laser beam expanded less than ten percent at high thermal loading.Further, in most cases, the laser rod sustained continuous pulsing atfive and ten pulses per second without breakage.

The phosphate laser glass of this invention thus provides substantialimprovements over the prior art, including high gain with athermalbehavior and good durability. A more detailed description of thepreferred embodiments of the laser glass composition of this inventionfollows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the phosphate laser glass composition ofthis invention may best be described by reference to specificexperimental examples which have been formulated and tested. Asdescribed above, the constituents of the base glass composition in thefollowing examples are given in mole percent to permit a one to onesubstitution of equivalent constituents and a direct comparison of thetest results. The additions, including the lasing constituent and theantisolarent, are given in weight percent of the total glass compositionbecause these constituents are not changed in most examples. In each ofthe following examples, a one kilogram melt was formulated and tested.Larger melts of the most preferred embodiments were then made, asdescribed below.

In addition to the examples given hereinbelow, a number of phosphatelaser glass compositions were made, tested and rejected because of laserbeam divergence or erratic behavior, poor water durability and/or poorlaser gain. It was thus determined that phosphate laser glasscompositions outside the claimed ranges did not possess the sameadvantages as the preferred embodiments. The tests were all conducted onlaser rods formed from the phosphate laser glass composition. The laserrods tested were cylindrical; 1/4 inch diameter and 31/4 inches inlength. The ends of the rods were polished plano-plano. The mirrors usedin the threshold test were 99.9% and 55% reflective dielectrics having acavity length of 200 mm. The flash lamp used in an ILC L-336 having a 3inch arc length and a pulse width of 110 microseconds. A water coolantwas used.

Water durability was determined by placing a preweighed and measuredsample of laser glass in a beaker of boiling distilled water andmeasuring the weight loss after 60 minutes. The results are reported inmillionths of grams per minute, per square centimeter. It wasexperimentally determined that a weight loss of less than 6×10⁻⁶gm/min/cm² was acceptable for water cooled laser systems.

The rod was installed in the laser system and the mirrors aligned. Aninitial measurement was taken to determine lasing threshold. Then, thelasing efficiency of the rod was determined at a constant pulserepetition rate of one pulse per second. The energy input was varied andthe corresponding lasing output measured up to a maximum input of 40.5joules.

Beam divergence was measured by firing the laser through a one meterfocal length lens at exposed Polaroid film located at the focal point ofthe lens. A direct comparison could then be made of the beam diameterand shape on the Polaroid film at one pulse per second, five pulses persecond and ten pulses per second. This method provides a directexperimental comparison of the thermal properties of the laser glasscompositions under high thermal loading. The following are given asExamples of phosphate laser glass compositions.

EXAMPLE 1

A base glass having the following composition in mole percent wasformulated:

    ______________________________________                                                    Mole % Weight %                                                   ______________________________________                                        P.sub.2 O.sub.5                                                                             65%      72.46%                                                 Li.sub.2 O    10%      2.35%                                                  BaO           12.5%    15.05%                                                 SrO           10%      8.14%                                                  Al.sub.2 O.sub.3                                                                            2.5%     2.00%                                                                100%     100.00%                                                ______________________________________                                    

A lasing component, Nd₂ O₃ and solarization inhibitors were added to thebase glass, as follows, in weight percent of the total composition:

    ______________________________________                                                Nd.sub.2 O.sub.3                                                                            3%                                                              Nb.sub.2 O.sub.5                                                                            1%                                                              Sb.sub.2 O.sub.3                                                                            0.5%                                                    ______________________________________                                    

The laser rod formed from this phosphate laser glass composition has alasing threshold of 8.8 joules and an output of 874 millijoules. Lasingthreshold, as used herein, is a measurement of the onset of lasing usinga 55 percent output reflector, as described above. The output is ameasurement of the gain of the laser in millijoules at 40.5 joulesinput. Water durability was experimentally determined, as describedabove, as 5×10⁻⁶ gm/min/cm², which is considered very acceptable.

The laser system utilized in these tests is described above. The cavitylength is 200 mm and the flash lamp was an ILC Technology IncorporatedL-336 having a 3-inch arc length and a flash lamp pulse width of 110microseconds. The pulse firing network was 25 microfarads at 50microhenrys. The cavity reflectors are silver-plated partial cylindersand the system utilizes water as the coolant.

The beam divergence was measured by firing the laser through a lens atexposed Polaroid film located at the focal length of the lens, asdescribed. In this Example, a slight instability was noted at fivepulses per second with a slight divergence of the beam. 261 shots werefired at ten pulses per second until lasing stopped.

EXAMPLE 2

A base glass having the following composition in mole percent wasformulated:

    ______________________________________                                                    Mole %     Weight %                                               ______________________________________                                        P.sub.2 O.sub.5                                                                             69%          74.75%                                             K.sub.2 O     5.0%         3.60%                                              BaO           13%          15.21%                                             CaO           10.5%        4.49%                                              Al.sub.2 O.sub.3                                                                            2.5%         1.95%                                                            100%         100.00%                                            ______________________________________                                    

A lasing component Nd₂ O₃ and solarization inhibitors Nb₂ O₅ and Sb₂ O₃were added to the base glass composition, as follows, in weight percentof the total composition:

    ______________________________________                                                Nd.sub.2 O.sub.3                                                                            3%                                                              Nb.sub.2 O.sub.5                                                                            1%                                                              Sb.sub.2 O.sub.3                                                                            0.5%                                                    ______________________________________                                    

The laser rod formed from this phosphate laser glass composition had alasing threshold of 8.6 joules and an output of 604 millijoules. Thelaser beam was found to be somewhat unstable at ten pulses per secondand broke following 361 shots. Water durability was experimentallydetermined to be 13.5×10⁻⁶ gm/min/cm², which is not acceptable forcommercial applications. This Example and all of the following Examplesuse the laser system described in regard to Example 1.

It was concluded from this Example that the substitution of K₂ O for Li₂O reduced the output or gain of the laser and resulted in furtherinstability of the laser beam. Example 2 thus confirmed that theaddition of lithia in Example 1 resulted in a lower expansion glass. Itwas thus decided to increase the concentration of BaO, which provides anegative temperature coefficient of index, dn/dt, as set forth inExample 3.

EXAMPLE 3

A base glass having the following composition in mole percent wasformulated:

    ______________________________________                                                    Mole %     Weight %                                               ______________________________________                                        P.sub.2 O.sub.5                                                                             65%          69.73%                                             Li.sub.2 O    10%          2.26%                                              BaO           22.5%        26.08%                                             Al.sub.2 O.sub.3                                                                            2.5%         1.93%                                                            100%         100.00%                                            ______________________________________                                    

A lasing component Nd₂ O₃ and solarization inhibitors Nb₂ O₅ and Sb₂ O₃were then added to the base glass, as follows, in weight percent of thetotal composition:

    ______________________________________                                                Nd.sub.2 O.sub.3                                                                            3%                                                              Nb.sub.2 O.sub.5                                                                            1%                                                              Sb.sub.2 O.sub.3                                                                            0.5%                                                    ______________________________________                                    

The laser rod formulated from this phosphate laser glass composition hada lasing threshold of 7.6 joules and an output of 1066 millijoules.Water durability was determined experimentally to be 7.5×10⁻⁶gm/min/cm², which is slightly greater than acceptable for commercialapplications. The laser beam had a slight expansion at five pulses persecond, but otherwise appeared to be stable. Comparing this Example withExample 1, it will be seen that the substitution of 22.5 mole percentBaO for 12.5 percent BaO plus 10 percent SrO substantially improved thegain and resulted in a lower lasing threshold. The beam however divergedat five pulses per second and had only 163 shots to cessation of lasing.Thus, it was decided to add K₂ O to provide a negative dn/dt.

EXAMPLE 4

A base glass having the following composition in mole percent wasformulated:

    ______________________________________                                                    Mole %     Weight %                                               ______________________________________                                        P.sub.2 O.sub.5                                                                             65%          71.57%                                             Li.sub.2 O    2.5%         0.60%                                              K.sub.2 O     5.0%         3.65%                                              BaO           15%          17.85%                                             CaO           10%          4.35%                                              Al.sub.2 O.sub.3                                                                            2.5%         1.98%                                                            100%         100.00%                                            ______________________________________                                    

A lasing constituent and solarization inhibitors were added to the baseglass, as follows, in weight percent of the total composition:

    ______________________________________                                                Nd.sub.2 O.sub.3                                                                            3%                                                              Nb.sub.2 O.sub.5                                                                            1%                                                              Sb.sub.2 O.sub.3                                                                            0.5%                                                    ______________________________________                                    

The laser rod formulated from this phosphate laser glass composition hada lasing threshold of 8.4 joules and an output of 758 millijoules. Waterdurability was determined experimentally to be 11.5×10⁻⁶ gm/min/cm²,which is not acceptable for commercial water cooled laser systems. Thelaser beam had a slight instability at ten pulses per second andprovided 363 shots before lasing discontinued. Comparing this Examplewith Example 2, it will be seen that a slightly better gain was achievedand the laser was more stable. It was decided to increase theconcentration of Al₂ O₃ and K₂ O to determine the effect of thesechanges.

EXAMPLE 5

A base glass having the following composition and mole percent wasformulated:

    ______________________________________                                                    Mole %     Weight %                                               ______________________________________                                        P.sub.2 O.sub.5                                                                             65%          70.66%                                             K.sub.2 O     6.5%         4.69%                                              BaO           15%          17.62%                                             CaO           10%          4.30%                                              Al.sub.2 O.sub.3                                                                            3.5%         2.73%                                                            100%         100.00%                                            ______________________________________                                    

A lasing component and solarization inhibitors were added to the baseglass, as follows, in weight percent of the total composition:

    ______________________________________                                                Nd.sub.2 O.sub.3                                                                            3%                                                              Nb.sub.2 O.sub.5                                                                            1%                                                              Sb.sub.2 O.sub.3                                                                            0.5%                                                    ______________________________________                                    

The laser rod formed from this phosphate laser glass composition had alasing threshold of 9.1 joules and an output of 780 millijoules. Thetests were repeated because of certain inconsistencies, resulting in athreshold of 8.8 joules and an output of 972 millijoules. Waterdurability was determined experimentally to be 11.5×10⁻⁶ gm/min/cm²,which is not acceptable for commercial water cooled laser systems. Thebeam was stable at one pulse per second, five pulses per second and tenpulses per second and lasing continued stable through 421 shots. Thelaser glass of this Example is therefore more stable and durable,however the glass does not fully achieve the objective of high gain. Itwas decided therefore to increase the concentration of K₂ O and returnto a greater concentration of BaO.

EXAMPLE 6

A base glass having the following composition in mole percent wasformulated:

    ______________________________________                                                    Mole %     Weight %                                               ______________________________________                                        P.sub.2 O.sub.5                                                                             65%          65.80%                                             K.sub.2 O     7.5%         5.04%                                              BaO           25%          27.34%                                             Al.sub.2 O.sub.3                                                                            2.5%         1.82%                                                            100%         100.00%                                            ______________________________________                                    

A lasing component and solarization inhibitors were added to the baseglass, as follows, in weight percent of the total composition:

    ______________________________________                                                Nd.sub.2 O.sub.3                                                                            3%                                                              Nb.sub.2 O.sub.5                                                                            1%                                                              Sb.sub.2 O.sub.3                                                                            0.5%                                                    ______________________________________                                    

The laser rod formulated from this phosphate laser glass composition hada lasing threshold of 8.8 joules and an output of only 532 millijoules.Water durability was determined experimentally to be 8×10⁻⁶ gm/min/cm²,which is greater than acceptable for commercial water cooled lasersystems. It was decided therefore to increase the concentration of Al₂O₃ to determine whether an increase would improve water durability. Thelaser beam was unstable at five and ten pulses per second and the roddiscontinued lasing, almost immediately. Comparing this Example withExample 5, it was decided to decrease the concentration of K₂ O andincrease the concentration of of Al₂ O₃ to determine whether thesechanges would increase the stability of the glass while increasing thegain.

EXAMPLE 7

A base glass having the following composition, in mole percent, wasformulated:

    ______________________________________                                                    Mole %     Weight %                                               ______________________________________                                        P.sub.2 O.sub.5                                                                             65%          65.77%                                             K.sub.2 O     6.5%         4.36%                                              BaO           25%          27.33%                                             Al.sub.2 O.sub.3                                                                            3.5%         2.54%                                                            100%         100.00%                                            ______________________________________                                    

A lasing component and solarization inhibitors were then added to thebase glass, as follows, in weight percent of the total composition:

    ______________________________________                                                Nd.sub.2 O.sub.3                                                                            3%                                                              Nb.sub.2 O.sub.5                                                                            1%                                                              Sb.sub.2 O.sub.3                                                                            0.5%                                                    ______________________________________                                    

The laser rod formed from this phosphate laser glass composition had alasing threshold of 8.8 joules and an output of 643 millijoules. Waterdurability was determined experimentally to be 5.5×10⁻⁶ gm/min/cm²,which is considered acceptable for water cooled laser systems. Theimprovement in water durability was apparently due to the increase inthe concentration of Al₂ O₃. The laser beam was unstable at five pulsesper second and lasing discontinued almost immediately. It was decidedtherefore to return to a glass composition similar to Example 4 whileadjusting the relative concentrations of BaO and CaO to determine theeffect of this change upon stability and gain.

EXAMPLE 8

A base glass having the following composition, in mole percent, wasformulated:

    ______________________________________                                                    Mole %     Weight %                                               ______________________________________                                        P.sub.2 O.sub.5                                                                             65%          70.26%                                             Li.sub.2 O    2.5%         0.57%                                              K.sub.2 O     5.0%         3.59%                                              BaO           17.5%        20.44%                                             CaO           7.5%         3.20%                                              Al.sub.2 O.sub.3                                                                            2.5%         1.94%                                                            100%         100.00%                                            ______________________________________                                    

A lasing component and solarization inhibitors were added to the baseglass, as follows, in weight percent of the total composition:

    ______________________________________                                                Nd.sub.2 O.sub.3                                                                            3%                                                              Nb.sub.2 O.sub.5                                                                            1%                                                              Sb.sub.2 O.sub.3                                                                            0.5%                                                    ______________________________________                                    

The laser rod formed from this phosphate laser glass composition had alasing threshold of 9.7 joules and an output of 643 millijoules. Waterdurability was determined experimentally to be 7.4×10⁻⁶ gm/min/cm²,which is slightly greater than acceptable for commercial water cooledlaser systems. The laser beam was stable, however a very slightexpansion was noted at five pulses per second. Thus, the addition of CaOimproved the stability of the glass, however the threshold was high andthe gain was low. The glass was relatively strong, continuing for 332shots before lasing discontinued. It was decided therefore to return toExample 3, but include both Li₂ O and K₂ O to improve the stability ofthe glass.

EXAMPLE 9

A base glass having the following composition, in mole percent, wasformulated:

    ______________________________________                                                    Mole %     Weight %                                               ______________________________________                                        P.sub.2 O.sub.5                                                                             65%          68.90%                                             Li.sub.2 O    7.5%         1.67%                                              K.sub.2 O     2.5%         1.76%                                              BaO           22.5%        25.77%                                             Al.sub.2 O.sub.3                                                                            2.5%         1.90%                                                            100%         100.00%                                            ______________________________________                                    

A lasing component and solarization inhibitors were then added to thebase glass, as follows, in weight percent of the total composition:

    ______________________________________                                                Nd.sub.2 O.sub.3                                                                            3%                                                              Nb.sub.2 O.sub.5                                                                            1%                                                              Sb.sub.2 O.sub.3                                                                            0.5%                                                    ______________________________________                                    

The laser rod formed from this phosphate laser glass composition had alasing threshold of 8.0 joules and an output of 880 millijoules. Waterdurability was experimentally determined to be 5.9×10⁻⁶ gm/min/cm²,which is considered acceptable for commercial water cooled lasersystems. A slight expansion in the laser beam was noted at five pulsesper second, but then became stable. A slight instability was also notedat ten pulses per second. It was decided therefore to increase theconcentration of BaO to 25% and eliminate the K₂ O to determine theeffect of these changes upon gain and stability.

EXAMPLE 10

A base glass having the following composition, in mole percent, wasformulated:

    ______________________________________                                                    Mole %     Weight %                                               ______________________________________                                        P.sub.2 O.sub.5                                                                             65%          68.15%                                             Li.sub.2 O    7.5%         1.66%                                              BaO           25%          28.31%                                             Al.sub.2 O.sub.3                                                                            2.5%         1.88%                                                            100%         100.00%                                            ______________________________________                                    

A lasing component and solarization inhibitors were then added to thebase glass, as follows, in weight percent of the total composition:

    ______________________________________                                                Nd.sub.2 O.sub.3                                                                            3%                                                              Nb.sub.2 O.sub.5                                                                            1%                                                              Sb.sub.2 O.sub.3                                                                            0.5%                                                    ______________________________________                                    

The laser rod formed from this phosphate laser glass composition had alasing threshold of 8.0 joules and an output of 791 millijoules. Waterdurability was determined experimentally to be 4×10⁻⁶ gm/min/cm², whichis considered very acceptable for commercial water cooled laser systems.The laser beam expanded at five pulses per second and was unstable atten pulses per second. The laser rod broke after 200 shots. It was thusdecided to increase the concentration of Li₂ O and Al₂ O₃ to obtain amore stable laser glass as set forth in the next example.

EXAMPLE 11

A base glass having the following composition, in mole percent, wasformulated:

    ______________________________________                                                    Mole %     Weight %                                               ______________________________________                                        P.sub.2 O.sub.5                                                                             65%          69.36%                                             Li.sub.2 O    9%           2.02%                                              BaO           22.5%        25.94%                                             Al.sub.2 O.sub.3                                                                            3.5%         2.68%                                                            100%         100.00%                                            ______________________________________                                    

A lasing component and solarization inhibitors were then added to thebase glass, as follows, in weight percent of the total composition:

    ______________________________________                                                Nd.sub.2 O.sub.3                                                                            3%                                                              Nb.sub.2 O.sub.5                                                                            1%                                                              Sb.sub.2 O.sub.3                                                                            0.5%                                                    ______________________________________                                    

The laser rod formed from this phosphate laser glass composition had alasing threshold of 8.4 joules and an output of 890 millijoules. Waterdurability was determined experimentally to be 3.7×10⁻⁶ gm/min/cm²,which is very acceptable for commercial water cooled laser systems. Thelaser continued for 267 shots before lasing discontinued. The laser beamexpanded and was slightly unstable at five pulses per second. It will benoted that the changes in the base glass composition resulted inimproved gain and stability. It was thus decided to attempt to combinethe advantages of Examples 3 and 9 by using both Li₂ O and K₂ O as setforth in the next Example.

EXAMPLE 12

A base glass having the following composition, in mole percent, wasformulated:

    ______________________________________                                                    Mole %     Weight %                                               ______________________________________                                        P.sub.2 O.sub.5                                                                             65%          69.23%                                             Li.sub.2 O    8.5%         1.91%                                              K.sub.2 O     1.5%         1.06%                                              BaO           22.5%        25.89%                                             Al.sub.2 O.sub.3                                                                            2.5%         1.91%                                                            100%         100.00%                                            ______________________________________                                    

A lasing component and solarization inhibitors were then added to thebase glass, as follows, in weight percent of the total composition:

    ______________________________________                                                Nd.sub.2 O.sub.3                                                                            3%                                                              Nb.sub.2 O.sub.5                                                                            1%                                                              Sb.sub.2 O.sub.3                                                                            0.5%                                                    ______________________________________                                    

The laser rod formed from this phosphate laser glass of composition hada lasing threshold of only 7.8 joules and an output of 1005 millijoules.Water durability was experimentally determined to be 5.5×10⁻⁶gm/min/cm², which is considered acceptable for commercial water cooledsystems. The laser beam was stable at one, five and ten pulses persecond. This glass composition was therefore considered one of the mostpreferred and promising compositions tested.

EXAMPLE 13

A base glass having the following composition, in mole percent, wasformulated:

    ______________________________________                                                    Mole %     Weight %                                               ______________________________________                                        P.sub.2 O.sub.5                                                                             66%          69.47%                                             Li.sub.2 O    7.5%         1.66%                                              K.sub.2 O     1.5%         1.05%                                              BaO           22.5%        25.93%                                             Al.sub.2 O.sub.3                                                                            2.5%         1.89%                                                            100%         100.00%                                            ______________________________________                                    

A lasing component and solarization inhibitors were then added to thebase glass, as follows, in weight percent of the total composition.

    ______________________________________                                                Nd.sub.2 O.sub.3                                                                            3%                                                              Nb.sub.2 O.sub.5                                                                            1%                                                              Sb.sub.2 O.sub.3                                                                            0.5%                                                    ______________________________________                                    

The laser rod formed from this phosphate laser glass composition had alasing threshold of 7.8 joules and a measured output of only 582millijoules. Water durability was determined experimentally to be4.7×10⁻⁶ gm/min/cm², which is very acceptable for commercial watercooled laser systems. The laser beam was slightly unstable at fivepulses per second and unstable at ten pulses per second. The low gainand instability can not be explained from the formulation and it isbelieved that either a mistake in formulation or testing occurred.

EXAMPLE 14

A base glass having the following composition, in mole percent, wasformulated:

    ______________________________________                                                    Mole %     Weight %                                               ______________________________________                                        P.sub.2 O.sub.5                                                                             65%          68.09%                                             Na.sub.2 O    10%          4.57%                                              BaO           22.5%        25.46%                                             Al.sub.2 O.sub.3                                                                            2.5%         1.88%                                                            100%         100.00%                                            ______________________________________                                    

A lasing component and solarization inhibitors were then added to thebase glass, as follows, in weight percent of the total composition:

    ______________________________________                                                Nd.sub.2 O.sub.3                                                                            3%                                                              Nb.sub.2 O.sub.5                                                                            1%                                                              Sb.sub.2 O.sub.3                                                                            0.5%                                                    ______________________________________                                    

The laser rod formed from this phosphate laser glass composition had alasing threshold of 8.0 joules and an output of only 582 millijoules.Water durability was experimentally determined to be 7.8×10⁻⁶gm/min/cm², which is not acceptable for commercial water cooled lasersystems. Further, the laser beam was unstable at five pulses per secondand substantially reduced at ten pulses per second. It was decidedtherefore that the substitution of Na₂ O for Li₂ O and K₂ O wasunacceptable, at least in this glass composition. It will be notedhowever that Na₂ O was added to Li₂ O in Example 16, as described below.

It was decided before to pursue minor variations in the glasscomposition of Example 12 by varying the relative concentration of Li₂ Oand K₂ O as set forth in the next Example.

EXAMPLE 15

A base glass having the following composition, in mole percent, wasformulated:

    ______________________________________                                                    Mole %     Weight %                                               ______________________________________                                        P.sub.2 O.sub.5                                                                             65%          69.38%                                             Li.sub.2 O    8.5%         1.91%                                              K.sub.2 O     2.0%         1.42%                                              BaO           22%          25.37%                                             Al.sub.2 O.sub.3                                                                            2.5%         1.92%                                                            100%         100.00%                                            ______________________________________                                    

A lasing component and solarization inhibitors were then added to thebase glass, as follows, in weight percent of the total composition:

    ______________________________________                                                Nd.sub.2 O.sub.3                                                                            3%                                                              Nb.sub.2 O.sub.5                                                                            1%                                                              Sb.sub.2 O.sub.3                                                                            0.5%                                                    ______________________________________                                    

The laser rod formed from this phosphate laser glass composition had alasing threshold of 8.8 joules and an output of 840 millijoules. Waterdurability was determined experimentally to be 5.2×10⁻⁶ gm/min/cm²,which is considered acceptable for commercial water cooled lasersystems. The laser beam was slightly unstable at five pulses per second.It was decided therefore to substitute Na₂ O for K₂ O and determine theeffect of this change.

EXAMPLE 16

A base glass having the following composition, in mole percent, wasformulated:

    ______________________________________                                                    Mole %     Weight %                                               ______________________________________                                        P.sub.2 O.sub.5                                                                             65%          69.49%                                             Li.sub.2 O    8.5%         1.91%                                              Na.sub.2 O    1.5%         0.70%                                              BaO           22.5%        25.98%                                             Al.sub.2 O.sub.3                                                                            2.5%         1.92%                                                            100%         100.00%                                            ______________________________________                                    

A lasing component and solarization inhibitors were then added to thebase glass, as follows, in weight percent of the total composition.

    ______________________________________                                                Nd.sub.2 O.sub.3                                                                            3%                                                              Nb.sub.2 O.sub.5                                                                            1%                                                              Sb.sub.2 O.sub.3                                                                            0.5%                                                    ______________________________________                                    

The laser rod formed from this phosphate laser glass composition had alasing threshold of 9 joules and an output of 750 millijoules. Waterdurability was determined experimentally to be 5.8×10⁻⁶ gm/min/cm²,which is considered acceptable for most commercial water cooled lasersystems. The laser beam was slightly unstable at ten pulses per second.No breakage occurred of the rod at 400 watts. The increase in the lasingthreshold and decrease in gain, however, indicated that, in thepreferred embodiment, Na₂ O can not be substituted for K₂ O in thephosphate laser glass of this invention.

EXAMPLE 17

A base glass having the following composition, in mole percent, wasformulated:

    ______________________________________                                                    Mole % Weight %                                                   ______________________________________                                        P.sub.2 O.sub.5                                                                             63%      67.69%                                                 Li.sub.2 O    8.6%     1.95%                                                  K.sub.2 O     1.5%     1.07%                                                  BaO           22.8%    26.47%                                                 CaO           3.1%     1.32%                                                  Al.sub.2 O.sub.3                                                                            1%       1.51%                                                                100%     100.00%                                                ______________________________________                                    

Eight percent by weight of the total composition of Nd₂ O₃ was added tothe base glass as the lasing component together with one weight percentNb₂ O₅ and 0.5 percent Sb₂ O₃. The laser rod formed from this phosphatelaser glass composition had a lasing threshold of 7.0 joules and a gainof 1310 millijoules. Water durability was experimentally determined tobe 4.5×10⁻⁶ gm/min/cm², which is very acceptable for commercial watercooled laser systems. The laser beam increased slightly at five pulsesper second, but remained stable. The laser rod did not break at 400watts.

This Example illustrates the fact that it is possible to substitute Nd₂O₃ for Al₂ O₃ and thereby improve the gain. It was decided therefore tocontinue this line of investigation in the following Examples.

EXAMPLE 18

A base glass having the following composition, in mole percent, wasformulated:

    ______________________________________                                                    Mole % Weight %                                                   ______________________________________                                        P.sub.2 O.sub.5                                                                             63%      67.69%                                                 Li.sub.2 O    8.6%     1.95%                                                  K.sub.2 O     1.5%     1.07%                                                  BaO           22.8%    26.47%                                                 CaO           3.1%     1.32%                                                  Al.sub.2 O.sub.3                                                                            1.%      1.51%                                                                100%     100.00%                                                ______________________________________                                    

Nine percent by weight of the total composition of Nd₂ O₃ was added tothe base glass composition as the lasing component. One weight percentNb₂ O₅ and 0.5 percent Sb₂ O₃ were added as solarization inhibitors. Thelaser rod formed from this phosphate laser glass composition had alasing threshold of only 6.7 joules and an output of 1050 millijoules. Aslight weight gain was measured experimentally in the test for waterdurability. It was determined that little or no loss resulted in thetest. The laser beam was stable, but the rod broke under continuedpulsing. This Example further illustrates the fact that it is possibleto substitute Nd₂ O₃ for Al₂ O₃, however the glass appears to be nearlysaturated with Nd₂ O₃ and has a somewhat reduced strength.

EXAMPLE 19

A base glass having the following composition, in mole percent, wasformulated:

    ______________________________________                                                    Mole %  Weight %                                                  ______________________________________                                        P.sub.2 O.sub.5                                                                             63.5%    68.59%                                                 Li.sub.2 O    8.7%     1.98%                                                  K.sub.2 O     1.6%     1.15%                                                  BaO           23.1%    26.97%                                                 CaO           3.1%     1.32%                                                                100%     100.00%                                                ______________________________________                                    

Eleven percent by weight of the total composition of Nd₂ O₃ was added tothe base glass as the lasing component and one weight percent Nb₂ O₅ and0.5 weight percent Sb₂ O₃ were added as solarization inhibitors. Thelaser rod formed from this phosphate laser glass composition had alasing threshold of 7.2 joules and an output of 1010 millijoules. Waterdurability was determined experimentally to be 8.5×10⁻⁶ gm/min/cm²,which is not acceptable for commercial water cooled laser systems. Thelaser rod further exhibited reduced strength and the glass appeared tobe saturated with Nd₂ O₃. It was decided therefore that the preferredphosphate laser glass composition should include at least one-half molepercent Al₂ O₃.

EXAMPLE 20

The following Example illustrates the affect of neodymium doping upondurability. A base glass having the same composition as Example 12 wasformulated, as follows:

    ______________________________________                                                    Mole % Weight %                                                   ______________________________________                                        P.sub.2 O.sub.5                                                                             65%      69.23%                                                 Li.sub.2 O    8.5%     1.91%                                                  K.sub.2 O     1.5%     1.06%                                                  BaO           22.5%    25.89%                                                 Al.sub.2 O.sub.3                                                                            2.5%     1.91%                                                                100%     100.00%                                                ______________________________________                                    

Seven percent by weight Nd₂ O₃ was then added to the base glasscomposition and solarization inhibitors were added, as follows, inweight percent of the total composition:

    ______________________________________                                                Nd.sub.2 O.sub.3                                                                            7%                                                              Nb.sub.2 O.sub.5                                                                            1%                                                              Sb.sub.2 O.sub.3                                                                            0.5%                                                    ______________________________________                                    

The laser glass rod formed from this phosphate laser glass compositionhad a threshold of 7.0 joules and an output or gain of 1070 millijouleswith an input of 40.5 joules, which compares favorably with the gain ofthe glass composition of Example 12. More importantly, the waterdurability was 3.7×10⁻⁶ gm/min/cm², which is substantially better thanExample 12 and which is considered very acceptable for commercial watercooled laser systems. Unfortunately, this glass exhibited instability atfive pulses per second, which stabilized at ten pulses per second. Thus,it was decided that Nd₂ O₃ should be substituted for Al₂ O₃, rather thanmerely increasing the concentration of Nd₂ O₃, which was experimentallyconfirmed in the following Examples.

EXAMPLE 21

A base glass having the following composition, in mole percent, wasformulated:

    ______________________________________                                                    Mole % Weight %                                                   ______________________________________                                        P.sub.2 O.sub.5                                                                             65.8%    69.87%                                                 Li.sub.2 O    8.6%     1.92%                                                  K.sub.2 O     1.5%     1.06%                                                  BaO           22.8%    26.16%                                                 Al.sub.2 O.sub.3                                                                            1.3%     0.99%                                                                100%     100.00%                                                ______________________________________                                    

Six weight percent (about 2.5 mole percent) Nd₂ O₃ and solarizationinhibitors were added to the base glass composition, as follows, inweight percent of the total composition:

    ______________________________________                                                Nd.sub.2 O.sub.3                                                                            6%                                                              Nb.sub.2 O.sub.5                                                                            1%                                                              Sb.sub.2 O.sub.3                                                                            0.5%                                                    ______________________________________                                    

The laser rod formed from the phosphate laser glass composition of thisExample had a lasing threshold of 7.0 joules and an output or gain of1000 millijoules. Water durability was determined experimentally to be6.6×10⁻⁶ gm/min/cm², which is greater than acceptable for commercialwater cooled laser systems.

The laser beam was slightly unstable at five and ten pulses per second,resulting in an oblong pattern on the film. The laser rod did not breakin the test. Based upon this experience, it was decided to eliminate theAl₂ O₃ and substitute Nd₂ O₃, as set forth in the following Example.

EXAMPLE 22

A base glass having the following composition, in mole percent, wasformulated.

    ______________________________________                                                    Mole % Weight %                                                   ______________________________________                                        P.sub.2 O.sub.5                                                                             64.6%    69.30%                                                 Li.sub.2 O    8.7%     1.92%                                                  K.sub.2 O     1.5%     1.06%                                                  CaO           2.1%     0.89%                                                  BaO           23.1%    26.77%                                                 Al.sub.2 O.sub.3                                                                            0%       0.00%                                                                100%     100.00%                                                ______________________________________                                    

Nine percent by weight Nd₂ O₃ was substituted for the Al₂ O₃, asfollows, in weight percent of the total composition:

    ______________________________________                                                Nd.sub.2 O.sub.3                                                                            9%                                                              Nb.sub.2 O.sub.5                                                                            1%                                                              Sb.sub.2 O.sub.3                                                                            0.5%                                                    ______________________________________                                    

The laser rod formed from this phosphate laser glass composition had alasing threshold of 8 joules and an output or gain of 690 millijouleswith an input of 40.5 joules. It was thus determined experimentally thatsubstitution of all of the Al₂ O₃ by Nd₂ O₃ resulted in a substantialreduction in gain. Thus, the most preferred embodiments include at least0.5 mole percent Al₂ O₃.

The water durability was determined experimentally for this laser glasscomposition to be 5.5×10⁻⁶ gm/min/cm², which is acceptable forcommercial water cooled laser systems. A slight instability in the laserbeam was noted at five pulses per second and the beam became unstable atten pulses per second. Based upon this instability of the laser beam, itwas decided to formulate a similar laser glass composition including Al₂O₃, which is found in the following Example.

EXAMPLE 23

A base glass having the following composition, in mole percent, wasformulated:

    ______________________________________                                                    Mole % Weight %                                                   ______________________________________                                        P.sub.2 O.sub.5                                                                             66.5%    70.51%                                                 Li.sub.2 O    8.5%     1.90%                                                  K.sub.2 O     1.5%     1.06%                                                  BaO           22.5%    25.77%                                                 Al.sub.2 O.sub.3                                                                            1.0%     0.76%                                                                100%     100.00%                                                ______________________________________                                    

The laser glass composition was doped with nine percent neodymium oxide,as follows, in weight percent of the total composition:

    ______________________________________                                                Nd.sub.2 O.sub.3                                                                            9%                                                              Nb.sub.2 O.sub.5                                                                            1%                                                              Sb.sub.2 O.sub.3                                                                            0.5%                                                    ______________________________________                                    

The laser rod formed from this phosphate laser glass composition had alasing threshold of 7.4 joules and an output or gain of 980 millijouleswith an input of 40.5 joules. Water durability was experimentallydetermined to be 2.9×10⁻⁶ gm/min/cm², which is excellent and quitesatisfactory for commercial water cooled laser glass systems. The laserbeam was quite stable at one, five and ten pulses per second. The laserglass of this composition was thus considered to be quite commercial.

EXAMPLE 24

This Example is given in weight percent because the glass composition isnot considered experimental and therefore it is unlikely that anysignificant modification or substitution will be made.

A laser glass having the following composition was made;

    ______________________________________                                        Glass           Glass      Base Glass                                         Weight %        Mole %     Mole %                                             ______________________________________                                        P.sub.2 O.sub.5                                                                       59.31%      58.5%      61.28%                                         Li.sub.2 O                                                                            1.92%       9.0%       9.43%                                          K.sub.2 O                                                                             1.06%       1.58%      1.65%                                          CaO     0.86%       2.15%      2.25%                                          BaO     25.94%      23.69%     24.80%                                         Al.sub.2 O.sub.3                                                                      0.41%       0.56%      0.59%                                          Nd.sub.2 O.sub.3                                                                      9.0%        3.75%      100%                                           Nb.sub.2 O.sub.5                                                                      1.0%        0.53%                                                     Sb.sub.2 O.sub.3                                                                      0.5%        0.24%                                                             100%        100%                                                      ______________________________________                                    

The laser rod formed from this glass composition had a lasing thresholdof 6.7 joules and a gain or output of 1225 millijoules with an input of40.5 joules. Water durability was experimentally determined to be 5×10⁻⁶gm/min/cm², which is considered very acceptable for commercial watercooled laser systems.

The laser beam was stable at one, five and ten pulses per second and thelaser rod did not break under repeated pulsing. Thus, the laser glasscomposition of this Example is considered to be very commercial.

It should be noted that the composition of Example 24 is the final oractual composition of the glass in weight percent, determined from theglass after casting. The previous Examples disclose the batchcomposition of the base glass in mole percent and the additives inweight percent. The mole percent of the cast glass of Example 24 wasthen calculated from the weight percent. Finally, the base glasscomposition was calculated in mole percent for comparison with theprevious Examples.

It has been determined from similar comparisons of the cast glasscomposition with the batch compositions that approximately fifteenpercent of the phosphorus pentoxide (P₂ O₅) was lost during melting andcasting. Thus, it was decided to add more of the P₂ O₅ as a complex,particularly barium phosphate, Ba(PO₃)₃, or lithium phosphate Li₃ PO₄ orpotassium phosphate, K₄ P₂ O₇. This reduced the P₂ O₅ and therefore theP₂ O₅ losses. By this method, it was found possible to reduce the P₂ O₅losses to about ten percent, or less. Ten weight percent of free P₂ O₅is therefore added to the base glass composition of the Examples aboveto make up for the loss.

Following the one kilogram experimental Examples above, larger melts, 4kg to 20 kg, were formulated to establish production processes andprocedures. During the initial larger melts, it was determined thatphosphate was volatilizing from the melts as described above. Thisphosphate loss shifted the base theoretical composition of the Examplesto a lower P₂ O₅ level, down about five mole percent, while raising thelevels of the other constituents in proportion to the individualtheoretical levels.

For example, a six kilogram melt of the composition of Example 12 wasformulated based upon the following batch composition, givenhereinabove:

    ______________________________________                                                Batch      Batch      Batch Weight %                                  Constituent                                                                           Mole %     Weight %   with additives                                  ______________________________________                                        P.sub.2 O.sub.5                                                                       65%        69.23%     66.12%                                          Li.sub.2 O                                                                            8.5%       1.91%      1.82%                                           K.sub.2 O                                                                             1.5%       1.06%      1.01%                                           BaO     22.5%      25.89%     24.73%                                          Al.sub.2 O.sub.3                                                                      2.5%       1.91%      1.82%                                           Nd.sub.2 O.sub.3              3%                                              Nb.sub.2 O.sub.5              1%                                              Sb.sub.2 O.sub.3              0.5%                                            ______________________________________                                    

After melting, the glass yield was weighed and, accounting for allincidental losses, it was determined that approximately 680 grams of P₂O₅ was lost through volatilization. This loss of P₂ O₅ resulted in thefollowing actual glass composition:

    ______________________________________                                                Glass     Glass     Base Glass                                                                             Base Glass                               Constituent                                                                           Weight %  Mole %    Weight % Mole %                                   ______________________________________                                        P.sub.2 O.sub.5                                                                       61.79%    59.30%    65.09%   60.63%                                   Li.sub.2 O                                                                            2.05%     9.35%     2.16%    9.56%                                    K.sub.2 O                                                                             1.14%     1.65%     1.22%    1.69%                                    BaO     27.88%    24.77%    29.37%   25.33%                                   Al.sub.2 O.sub.3                                                                      2.05%     2.74%     2.16%    2.80%                                    Nd.sub.2 O.sub.3                                                                      3.38%     1.36%                                                       Nb.sub.2 O.sub.5                                                                      1.13%     0.58%                                                       Sb.sub.2 O.sub.3                                                                      0.56%     0.26%                                                       ______________________________________                                    

It was experimentally determined that the loss of phosphate throughvolatilization is affected by composition, raw materials, formulationand processing time and temperature. As described above, this loss maybe controlled by using phosphate compounds, such as barium phosphate,lithium phosphate, etc. Further, in the Examples above, the P₂ O₅concentration in the final laser glass is about ten percent less thanthe batch composition, which is a loss of about five mole percent.

The preferred raw materials for the glass composition of this inventioninclude anhydrous phosphorus pentoxide, lithium orthophosphate,potassium pyrophosphate, barium metaphosphate, barium fluoride, aluminumoxide, aluminum fluoride, aluminum metaphosphate, calcium pyrophosphate,aluminum phosphate, strontium fluoride, magnesium and the remainder ofthe constituents preferably in oxide form, although they may also befluorides of phosphates.

In the preferred embodiments, the raw materials are reduced to moltencondition in a quartz crucible, adding small amounts of silica to themelt. The melt is then homogenized and refined in a platinum crucible.In the melting process, fluorine gas is evolved, particularly in thequartz crucible. Oxygen gas may then be introduced into the melt toconvert the fluorides to oxides. The molten glass is then cast, usingstandard techniques and annealed.

The following conclusions may be drawn from the Examples above. K₂ O andBaO appear to be major factors in lowering or providing a negativedn/dt, however increasing the concentration of K₂ O above about threemole percent appears to adversely affect the strength of the laser rod.Additions of SrO, CaO and MgO, in decending order, further reduce thetemperature coefficient of index, however Al₂ O₃ is highly positive.Further, the addition of lithia reduces the output or gain of the glass.Thus, the most preferred embodiment of the phosphate laser glass of thisinvention includes both Li₂ O and K₂ O, but includes a greaterpercentage of Li₂ O and a relatively small percent of K₂ O. The glasscomposition preferably includes about 20 to 30 mole percent BaO, howeverup to about 10 mole percent CaO or SrO may be substituted for BaO,provided the composition includes at least about 10 to 15 mole percentBaO. The preferred composition also includes at least 0.5 mole percentAl₂ O₃. It is noted however that Nd₂ O₃ may be substituted for Al₂ O₃,but the substitution is not direct in the above Examples and thepreferred compositions include a relatively narrow range of Al₂ O₃ plusNd₂ O₃, as described below.

The relation between the gain of a phosphate laser glass and waterdurability was found experimentally to be dependent upon theconcentrations of Al₂ O₃ and Nd₂ O₃ in a complex relation. In general,the principles controlling gain and water durability are as follows.Increases in the concentration of Al₂ O₃ improves the water durabilityof the phosphate laser glass, but tends to decrease the gain. Anincrease in the Nd₂ O₃ concentration decreases the fluorescent lifetimeof a phosphate laser glass, thus tending to decrease the gain. However,an increase the neodymium concentration also increases the absorption bythe laser glass of the flashlamp pump light, thereby tending to increasethe gain of the laser glass.

It has been discovered that there exists a level of neodymium oxideconcentration where the increase in absorption of the flashlamp pumplight is offset by the decrease in fluorescent lifetime due toconcentration quenching. This level, or point of diminishing return, isindicated by that point wherein the gain of the laser glass no longerincreases with an increase in neodymium oxide concentration. It wasdetermined experimentally that the gain of a phosphate laser glasshaving the general composition herein disclosed increases with increasesin the consentration of neodymium oxide until the concentration reachesabout ten percent by weight, when the gain decreases due to the decreasein the fluorescent lifetime. The fluorescent lifetime of the phosphatelaser glass decreases in a generally straight line function from about350 microseconds at two weight percent Nd₂ O₃ to less than 100microseconds at twelve weight percent Nd₂ O₃.

In order to maintain an acceptable water durability, it has beendetermined experimentally that the loss due to water should not begreater than about 6×10⁻⁶ gm/min/cm² at 100° C. It has now beendetermined that the sum of the concentrations of Al₂ O₃ and Nd₂ O₃should be at least 2.5 mole percent and less than about 5.5 molepercent. More preferably, the sum of the concentrations of Al₂ O₃ andNd₂ O₃ is four mole percent plus or minus one percent. Thisconcentration was determined experimentally as follows.

The water durability of Example 12 was 5.5×10⁻⁶ gm/min/cm², which isacceptable for commercial water cooled laser systems. Example 12includes 2.5 mole percent Al₂ O₃ and three weight percent Nd₂ O₃. Threeweight percent Nd₂ O₃ equals about 0.6 mole percent. Thus, the sum ofAl₂ O₃ and Nd₂ O₃ in Example 12 is 3.1 mole percent. It has also beendetermined experimentally that this combination of Al₂ O₃ plus Nd₂ O₃ isimportant to maintaining a stable laser beam. The laser glasscomposition of Example 12 has a stable beam, however a beam divergencewas noted in the composition of Example 17 which includes eight percentby weight of the total composition of Nd₂ O₃. Glass compositions having2.5 mole percent Al₂ O₃ plus six weight percent Nd₂ O₃ were also foundto be unstable. Glass compositions having 2.5 percent Al₂ O₃ and two tofour weight percent Nd₂ O₃ were found stable. Example 24, which has astable beam, and is water durable, has 0.56 mole percent Al₂ O₃, plus3.75 mole percent Nd₂ O₃. In order to stabilize the laser beamdivergence at nine weight percent Nd₂ O₃, it was thus found necessary todecrease the concentration of Al₂ O₃. This decrease also had the desiredeffect of augmenting an increase in gain. Further, the increase in theconcentration of Nd₂ O₃ improved the water durability of the laser glasssufficiently to permit a reduction in the concentration of Al₂ O₃.Examples 23 and 24 illustrate good commercial glass compositions havinga high concentration of Nd₂ O₃. At the opposite end of the scale, theconcentration of Al₂ O₃ must be increased to about three mole percentwhere low concentrations of Nd₂ O₃ are used to maintain laser beamstability and good water durability.

Based upon these conclusions, the base glass of the phosphate laserglass of this invention includes the following, in mole percent: P₂ O₅55 to 70%, Li₂ O and K₂ O 3 to 15%, BaO 10 to 30%, CaO 0 to 15%, SrO 0to 15%, Al₂ O₃ 0.5 to 5% wherein the total of BaO, CaO and SrO is atleast 15 mole percent and the total of Al₂ O₃ plus Nd₂ O₃ is 2.5 to 5.5mole percent. The phosphate laser glass also includes a lasingcomponent, preferably Nd₂ O₃ and a solarization inhibitor. As describedabove, the preferred laser glass compositions include Li₂ O and agreater concentration of R₂ O, as follows, in mole percent: P₂ O₅ 55 to65%, Li₂ O 3 to 15%, K₂ O 0 to 10%, BaO 12 to 28%, CaO 0 to 15%, SrO 0to 15%, Al₂ O₃ 0.5 to 4%, wherein the total of BaO, CaO and SrO is 25 to30% and the total of Al₂ O₃ plus Nd₂ O₃ is four mole percent ± 1%. Asdescribed above, the total glass composition includes a lasing componentand a solarization inhibitor. More preferably, the base glasscomposition comprises the following, in mole percent: P₂ O₅ 55 to 65%,Li₂ O 6 to 12%, K₂ O 0 to 10%, BaO 15 to 28%, CaO 0 to 10%, SrO 0 to10%, Al₂ O₃ 1 to 4%, wherein the total Li₂ O plus K₂ O is 7 to 15 molepercent and the total BaO, CaO and SrO is 20 to 28% and the total Al₂ O₃plus Nd₂ O₃ is 4 ± 1 mole percent. The total glass composition includes0.5 to 10 percent, by weight of the lasing component, preferably Nd₂ O₃.Finally, the most preferred base glass composition includes thefollowing, in mole percent: P₂ O₅ 55 to 65%, Li₂ O 6 to 12%, K₂ O 1 to3%, BaO 20 to 26%, CaO 0 to 10%, SrO 0 to 10%, Al₂ O₃ 0.5 to 3%, whereinCaO and SrO may be substituted for BaO in concentrations up to 10percent and the total glass composition includes 0.5 to 10 percent, byweight of a lasing component, preferably Nd₂ O₃ and 0.5 to 5 percent ofan antisolarent, preferably selected from the group consisting of Sb₂O₃, Nb₂ O₅ and SiO₂. Nd₂ O₃ may be substituted for Al₂ O₃ in greaterconcentrations and the total Al₂ O₃ and Nd₂ O₃ is 4 ± 1 mole percent. Asdescribed above, up to about ten percent of the phosphate may be lostthrough volatilization.

As described above, the phosphate laser glass of this invention has ahigh gain, providing the most efficient operation possible, athermalbehavior, providing constant beam divergence, good durability,permitting water cooling and high thermal loading. Laser rods formedfrom the glass compositions are temperature stable, even under highthermal loading and provide excellent gain at a relatively low lasingthreshold. For example, in the laser system described, the laser rodshad a threshold of 9 joules or less with an output of over 800millijoules with an input of 40.5 joules at pulse repitition of onepulse per second. The most preferred embodiments had an output of over1000 millijoules and a stable laser beam.

We claim:
 1. A temperature stable phosphate laser glass comprising aphosphate base glass composition and additions including 0.5 to 10percent by weight of a lasing component and 0.5 to 5 percent by weightof a solarization inhibitor selected from the group consisting of SiO₂,Nb₂ O₅ and Sb₂ O₃, said phosphate base glass composition consistingessentially of the following, in mole percent:

    ______________________________________                                               P.sub.2 O.sub.5                                                                             55-65%                                                          Li.sub.2 O    6-12%                                                           K.sub.2 O     0-10%                                                           BaO           20-30%                                                          CaO           0-10%                                                           SrO           0-10%                                                           Al.sub.2 O.sub.3                                                                            1-4%                                                     ______________________________________                                    

wherein the total of Li₂ O and K₂ O is less than 15 mole percent and thetotal of Al₃ O₃ plus Nd₂ O₃ as the lasing component is 4 ± 1.5 molepercent.
 2. The temperature stable phosphate laser glass compositiondefined in claim 1, characterized in that said phosphate base glasscomposition includes one to three mole percent K₂ O and the total of Li₂O and K₂ O is 8 to 15 mole percent.
 3. The temperature stable phosphatelaser glass composition defined in claim 2, in the form of a laser rod,wherein the lasing threshold is less than 9 joules using a 55 percentoutput reflector and said laser rod is chemically stable using a watercoolant.
 4. The temperature stable phosphate laser glass defined inclaim 3, wherein the output of said laser rod is greater than 800millijoules with an input of 40.5 joules when pulsed at one pulse persecond.
 5. The temperature stable phosphate laser glass defined in claim4, characterized in that the laser beam generated by said laser rodexpands less than ten percent at ten pulses per second.
 6. The phosphatelaser glass composition defined in claim 1, characterized in that saidadditions comprise one to ten percent by weight of the total glasscomposition of Nd₂ O₃ and 0.5 to 3 percent by weight of the total glasscomposition of solarization inhibitors selected from the groupconsisting of SiO₂, Nb₂ O₅ and Sb₂ O₃.
 7. A temperature stable phosphatelaser glass, comprising a phosphate base glass composition and additionsincluding 0.5 to 10 percent by weight of a lasing component and 0.5 to 5percent by weight of a solarization inhibitor selected from the groupconsisting of SiO₂, Nb₂ O₅ and Sb₂ O₃, said base glass compositionconsisting essentially of the following, in mole percent:

    ______________________________________                                               P.sub.2 O.sub.5                                                                             55-65%                                                          Li.sub.2 O    6-12%                                                           K.sub.2 O     1-3%                                                            BaO           20-25%                                                          CaO           0-10%                                                           SrO           0-10%                                                           Al.sub.2 O.sub.3                                                                            .5-4%                                                    ______________________________________                                    

wherein up to ten mole percent of CaO, MgO and SrO are substituted forBaO and the total of Al₂ O₃ plus Nd₂ O₃ as the lasing component is 4 ±1.5 mole percent.
 8. The temperature stable phosphate laser glasscomposition defined in claim 16, characterized in that said glasscomposition is in the form of a laser rod having a lasing threshold ofless than 9 joules using a 55 percent output reflector and said laserrod has an output of greater than 800 millijoules at 40.5 joules inputwhen pulsed at one pulse per second.
 9. A neodymium doped temperaturestable phosphate laser glass composition, comprising a phosphate baseglass composition and additions including 0.5 to 10 percent by weight ofNd₂ O₃ and 0.5 to 5 percent by weight of a solarization inhibitorselected from the group consisting of SiO₂, Nb₂ O₅ and Sb₂ O₃, said baseglass composition consisting essentially of the following, in molepercent:

    ______________________________________                                               P.sub.2 O.sub.5                                                                             55-65%                                                          Li.sub.2 O    6-9%                                                            K.sub.2 O     1-3%                                                            BaO           20-25%                                                          CaO           0-10%                                                           SrO           0-10%                                                           Al.sub.2 O.sub.3                                                                            .5-3%                                                    ______________________________________                                    

wherein up to ten mole percent of CaO and SrO are substituted for BaOand the total of Al₂ O₃ plus Nd₂ O₃ is 4 ± 1.5 mole percent.
 10. Atemperature stable phosphate laser glass composition, comprising aphosphate base glass composition and additions including 0.5 to 10percent Nd₂ O₃ and 0.5 to 3 percent of a solarization inhibitor selectedfrom the group consisting of SiO₂, Nb₂ O₅ and Sb₂ O₃, said phosphatebase glass composition consisting essentially of the following, in molepercent:

    ______________________________________                                               P.sub.2 O.sub.5                                                                             65%                                                             Li.sub.2 O    8.5%                                                            K.sub.2 O     1.5%                                                            BaO           22.5%                                                           Al.sub.2 O.sub.3                                                                            2.5%                                                     ______________________________________                                    


11. A temperature stable phosphate laser glass composition, comprising aphosphate base glass composition and additions including 0.5 to 10percent Nd₂ O₃ and 0.5 to 3 percent of a solarization inhibitor selectedfrom the group consisting of SiO₂, Nb₂ O₅ and Sb₂ O₃, said phosphatebase glass composition consisting essentially of the following, in molepercent:

    ______________________________________                                               P.sub.2 O.sub.5                                                                             65%                                                             Li.sub.2 O    7.5%                                                            K.sub.2 O     2.5%                                                            BaO           22.5%                                                           Al.sub.2 O.sub.3                                                                            2.5%                                                     ______________________________________                                    


12. A temperature stable phosphate laser glass composition, comprising aphosphate base glass composition and additions including 0.5 to 10percent Nd₂ O₃ and 0.5 to 3 percent of a solarization inhibitor selectedfrom the group consisting of SiO₂, Nb₂ O₅ and Sb₂ O₃, said phosphatebase glass composition consisting essentially of the following, in molepercent:

    ______________________________________                                               P.sub.2 O.sub.5                                                                             65%                                                             Li.sub.2 O    8.5%                                                            K.sub.2 O     2.0%                                                            BaO           22%                                                             Al.sub.2 O.sub.3                                                                            2.5%                                                     ______________________________________                                    


13. A temperature stable phosphate laser glass composition, comprising aphosphate base glass composition and additions including 0.5 to 10percent Nd₂ O₃ and 0.5% to 3 percent of a solarization inhibitorselected from the group consisting of SiO₂, Nb₂ O₅ and Sb₂ O₃, saidphosphate base glass composition consisting essentially of thefollowing, in mole percent:

    ______________________________________                                               P.sub.2 O.sub.5                                                                             63%                                                             Li.sub.2 O    8.6%                                                            K.sub.2 O     1.5%                                                            BaO           22.8%                                                           CaO           3.1%                                                            Al.sub.2 O.sub.3                                                                            1%                                                       ______________________________________                                    


14. A temperature stable phosphate laser glass composition, comprising aphosphate base glass composition and additions including 0.5 to 10percent by weight Nd₂ O₃ and 0.5 to 3% by weight of a solarizationinhibitor selected from the group consisting of SiO₂, Nb₂ O₅ and Sb₂ O₅,said phosphate base glass composition consisting essentially of thefollowing, in mole percent:

    ______________________________________                                               P.sub.2 O.sub.5                                                                             66.5%                                                           Li.sub.2 O    8.5%                                                            K.sub.2 O     1.5%                                                            BaO           22.5%                                                           Al.sub.2 O.sub.3                                                                            1%                                                       ______________________________________                                    


15. A temperature stable phosphate laser glass composition consistingessentially of the following, in weight percent:

    ______________________________________                                               P.sub.2 O.sub.5                                                                             59%                                                             Li.sub.2 O    2%                                                              K.sub.2 O     1%                                                              CaO           1%                                                              BaO           26%                                                             Al.sub.2 O.sub.3                                                                            0.5%                                                            Nb.sub.2 O.sub.5                                                                            1%                                                              Sb.sub.2 O.sub.3                                                                            0.5%                                                            Nd.sub.2 O.sub.3                                                                            9%                                                       ______________________________________                                    